package org.apache.lucene.store;

/**
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

import java.io.IOException;

/**
 * Abstract base class for input from a file in a {@link Directory}. A
 * random-access input stream. Used for all Lucene index input operations.
 * 
 * @see Directory
 */
public abstract class IndexInput implements Cloneable {
	private char[] chars; // used by readString()

	/**
	 * Reads and returns a single byte.
	 * 
	 * @see IndexOutput#writeByte(byte)
	 */
	public abstract byte readByte() throws IOException;

	/**
	 * Reads a specified number of bytes into an array at the specified offset.
	 * 
	 * @param b
	 *            the array to read bytes into
	 * @param offset
	 *            the offset in the array to start storing bytes
	 * @param len
	 *            the number of bytes to read
	 * @see IndexOutput#writeBytes(byte[],int)
	 */
	public abstract void readBytes(byte[] b, int offset, int len)
			throws IOException;

	/**
	 * Reads a specified number of bytes into an array at the specified offset
	 * with control over whether the read should be buffered (callers who have
	 * their own buffer should pass in "false" for useBuffer). Currently only
	 * {@link BufferedIndexInput} respects this parameter.
	 * 
	 * @param b
	 *            the array to read bytes into
	 * @param offset
	 *            the offset in the array to start storing bytes
	 * @param len
	 *            the number of bytes to read
	 * @param useBuffer
	 *            set to false if the caller will handle buffering.
	 * @see IndexOutput#writeBytes(byte[],int)
	 */
	public void readBytes(byte[] b, int offset, int len, boolean useBuffer)
			throws IOException {
		// Default to ignoring useBuffer entirely
		readBytes(b, offset, len);
	}

	/**
	 * Reads four bytes and returns an int.
	 * 
	 * @see IndexOutput#writeInt(int)
	 */
	public int readInt() throws IOException {
		return ((readByte() & 0xFF) << 24) | ((readByte() & 0xFF) << 16)
				| ((readByte() & 0xFF) << 8) | (readByte() & 0xFF);
	}

	/**
	 * Reads an int stored in variable-length format. Reads between one and five
	 * bytes. Smaller values take fewer bytes. Negative numbers are not
	 * supported.
	 * 
	 * @see IndexOutput#writeVInt(int)
	 */
	public int readVInt() throws IOException {
		byte b = readByte();
		int i = b & 0x7F;
		for (int shift = 7; (b & 0x80) != 0; shift += 7) {
			b = readByte();
			i |= (b & 0x7F) << shift;
		}
		return i;
	}

	/**
	 * Reads eight bytes and returns a long.
	 * 
	 * @see IndexOutput#writeLong(long)
	 */
	public long readLong() throws IOException {
		return (((long) readInt()) << 32) | (readInt() & 0xFFFFFFFFL);
	}

	/**
	 * Reads a long stored in variable-length format. Reads between one and nine
	 * bytes. Smaller values take fewer bytes. Negative numbers are not
	 * supported.
	 */
	public long readVLong() throws IOException {
		byte b = readByte();
		long i = b & 0x7F;
		for (int shift = 7; (b & 0x80) != 0; shift += 7) {
			b = readByte();
			i |= (b & 0x7FL) << shift;
		}
		return i;
	}

	/**
	 * Reads a string.
	 * 
	 * @see IndexOutput#writeString(String)
	 */
	public String readString() throws IOException {
		int length = readVInt();
		if (chars == null || length > chars.length)
			chars = new char[length];
		readChars(chars, 0, length);
		return new String(chars, 0, length);
	}

	/**
	 * Reads UTF-8 encoded characters into an array.
	 * 
	 * @param buffer
	 *            the array to read characters into
	 * @param start
	 *            the offset in the array to start storing characters
	 * @param length
	 *            the number of characters to read
	 * @see IndexOutput#writeChars(String,int,int)
	 */
	public void readChars(char[] buffer, int start, int length)
			throws IOException {
		final int end = start + length;
		for (int i = start; i < end; i++) {
			byte b = readByte();
			if ((b & 0x80) == 0)
				buffer[i] = (char) (b & 0x7F);
			else if ((b & 0xE0) != 0xE0) {
				buffer[i] = (char) (((b & 0x1F) << 6) | (readByte() & 0x3F));
			} else
				buffer[i] = (char) (((b & 0x0F) << 12)
						| ((readByte() & 0x3F) << 6) | (readByte() & 0x3F));
		}
	}

	/**
	 * Expert
	 * 
	 * Similar to {@link #readChars(char[], int, int)} but does not do any
	 * conversion operations on the bytes it is reading in. It still has to
	 * invoke {@link #readByte()} just as {@link #readChars(char[], int, int)}
	 * does, but it does not need a buffer to store anything and it does not
	 * have to do any of the bitwise operations, since we don't actually care
	 * what is in the byte except to determine how many more bytes to read
	 * 
	 * @param length
	 *            The number of chars to read
	 */
	public void skipChars(int length) throws IOException {
		for (int i = 0; i < length; i++) {
			byte b = readByte();
			if ((b & 0x80) == 0) {
				// do nothing, we only need one byte
			} else if ((b & 0xE0) != 0xE0) {
				readByte();// read an additional byte
			} else {
				// read two additional bytes.
				readByte();
				readByte();
			}
		}
	}

	/** Closes the stream to futher operations. */
	public abstract void close() throws IOException;

	/**
	 * Returns the current position in this file, where the next read will
	 * occur.
	 * 
	 * @see #seek(long)
	 */
	public abstract long getFilePointer();

	/**
	 * Sets current position in this file, where the next read will occur.
	 * 
	 * @see #getFilePointer()
	 */
	public abstract void seek(long pos) throws IOException;

	/** The number of bytes in the file. */
	public abstract long length();

	/**
	 * Returns a clone of this stream.
	 * 
	 * <p>
	 * Clones of a stream access the same data, and are positioned at the same
	 * point as the stream they were cloned from.
	 * 
	 * <p>
	 * Expert: Subclasses must ensure that clones may be positioned at different
	 * points in the input from each other and from the stream they were cloned
	 * from.
	 */
	public Object clone() {
		IndexInput clone = null;
		try {
			clone = (IndexInput) super.clone();
		} catch (CloneNotSupportedException e) {
		}

		clone.chars = null;

		return clone;
	}

}
